Conventional propulsion devices for ships each include one spiral propeller. However, a propulsion device having one propeller cannot use rotation energy of a waster current caused by rotation of the propeller as a propulsive force and thus has a large energy loss.
A counter rotating propeller (CRP) may recover the rotation energy that is lost in this way as the propulsive force. In the CRP, two propellers installed in the same axial line generate the propulsive force while being rotated in opposite directions. Rotation energy of a fluid that passes through a front propeller is recovered by a rear propeller while the rear propeller is rotated in a reverse direction. Thus, the CRP may have excellent propulsion performance compared to the propulsion device having one propeller.
However, since the CRP includes an inverse rotation unit that implements opposite rotation of two propeller and a hollow shaft, it is relatively difficult to manufacture and install the CRP, and a high technology level is required so that the CRP operates stably while maintaining reliability.
U.S. Patent Laid-open Publication No. US2011/0033296 (published on Feb. 10, 2011) and Japanese Patent Laid-open Publication No. S62-279189 (published on Dec. 4, 1987) each disclose an example of the above-described CRP. U.S. Patent Laid-open Publication No. US2011/0033296 discloses a CRP having an epicyclic gear type inverse rotation unit and a hollow shaft, and Japanese Patent Laid-open Publication No. S62-279189 discloses a CRP in which an epicyclic gear type inverse rotation unit is installed at an end of a ship.